In an optical head device widely used recently, for recording/reproducing data with respect to an optical recording medium such as CD or DVD as an optical disk or a magneto-optical disk, pit data having different reflectances or concavity and convexity as a one-dimensional data row are recorded in a single recording surface of the optical recording medium or the recorded data are reproduced from it. Further, there has been attempted to increase the volume of data by laminating surfaces for recording and reproducing. However, the maximum number of lamination per a single disk was about four layers. Accordingly, there was a limit in improving the volume of recording.
On the other hand, the holographic recording for recording data in an optical recording medium by utilizing holography is usually carried out by superimposing an object light carrying image data and a reference light in the recording medium to produce an interference pattern and writing the produced interference pattern as a volume hologram in the optical recording medium. In reproducing the recorded data, a reproducing light is irradiated to the optical recording medium whereby image data are reproduced as a data light due to diffraction by the recorded interference pattern (see, for example, JP-A-2002-123949, hereinbelow, referred to as Patent Document 1).
In such technique, the interference pattern is written three-dimensionally as a volume hologram in the direction of thickness of the optical recording medium. Since it can increase significantly the recording capacity, it draws public attention recently. In the optical recording/reproducing system proposed in the optical data recording/reproducing device disclosed in the above-mentioned Patent Document 1 in particular, an optical disk having an optical recording layer and an optical reflection layer is used as an optical recording medium, a two-parted optical rotation plate and a single objective lens are used and an object light and a reference light are irradiated to the optical recording medium. Accordingly, the optical head device can have the same structure as for CD, DVD or the like. Therefore, the system is effective to reduce the size of the optical data recording/reproducing device.
With reference to FIG. 17, the function of the optical data recording/reproducing device as well as the two-parted optical rotation plate used therein will be described.
The optical data recording/reproducing device has a two-parted optical rotation plate 100 which comprises a two-parted optical rotation plate part 100R located at a right side (+X) of the optical axis and a two-parted optical rotation plate part 100L located at a left side (−X) of the optical axis. The two-parted optional rotation plate part 100R rotates polarization directions of an object light and a reference light by −45° and the two-parted optical rotation plate part 100L rotates the polarization directions by +45°. In determining signs of positive and negative with respect to the angle of rotation, the clockwise rotation is made positive in the coordinate system shown in FIG. 19.
In this optical data recording/reproducing device, an objective lens 118 is provided adjacent to the two-parted optical rotation plate 100 and the objective lens 118 is located facing the side of the optical recording layer 10A of the optical recording medium 10. Further, the optical data recording/reproducing device is provided with an actuator (not shown) for moving the objective lens 118 in the direction of thickness and the direction of tracks of the optical recording medium 10.
Here, definitions of the polarization directions will be made with reference to FIG. 19. The sign of an angle in a clockwise direction is to be positive. An A polarized light is a linearly polarized light which is formed by rotating an S polarized light along the Y axis direction by −45° or by rotating a P polarized light along the X axis direction by +45°, and a B polarized light is a linearly polarized light formed by rotating the S polarized light by +45° or the P polarized light by −45°. The A polarized light and the B polarized light have polarization directions perpendicular to each other.
(I) Then, the principle of recording the data using the two-parted optical rotation plate 100 will be explained.
An object light as an S polarized light is reflected at a polarizing beam splitter (PBS) 116 as a beam combining means to be incident into the two-parted optical rotation plate 100. A reference light as a P polarized light transmits through the polarizing beam splitter 116 to be incident into the two-parted optical rotation plate 100. Here, the object light 20R passing through the two-parted optical rotation plate part 100R and entering into the optical recording medium 10 becomes an A polarized light. On the other hand, the reference light 30L passing through the two-parted optical rotation plate part 100L and entering into the optical recording medium 10 becomes also an A polarized light.
The reference light 30L of A polarized light is reflected at the reflection plane 10D in the light reflection layer 10C of the optical recording medium 10 and passes through the same region as the object light 20R of A polarized light in the optical recording layer 10A. Since these object light 20R and reference light 30L have polarization directions agreed with each other, an interference pattern is formed by the interference of these lights. Further, the object light 20R of A polarized light is reflected at the reflection plane 10D of the optical recording medium 10, and it passes through the same region as the reference light 30L of A polarized light in the optical recording layer 10A. These object light 20R and reference light 30L also produce an interference pattern by the interference since their polarization directions agree with each other.
Accordingly, in the optical recording layer 10A, the interference pattern due to the interference of the A polarized object light 20R before entering into the reflection plane 10D with the A polarized reference light 30L after being reflected at the reflection plane 10D and the interference pattern due to the interference of the A polarized reference light 30L before entering into the reflection plane 10D with the A polarized object light 10R after being reflected at the reflection plane 10D are recorded in a three-dimensional manner. Here, a transparent substrate 10B is formed between the reflection plane 10D and the data recording medium layer 10A.
Similarly, the object light 20L having passed through the two-parted optical rotation plate part 100L to enter into the optical recording medium 10 becomes a B polarized light. Further, the reference light 30R having passed through the two-parted optical rotation plate part 100R to enter into the optical recording medium 10 becomes also a B polarized light. The B polarized reference light 30R is reflected at the reflection plane 10D of the optical recording medium 10 and passes through the same region as the B polarized object light 20L in the optical recording layer 10A.
Since these object light 20L and reference light 30R have their agreed polarization directions, an interference pattern is produced by interference. Further, the B polarized object light 20L is reflected at the reflection plane 10D of the optical recording medium 10 and passes through the same region as the B polarized reference light 30R in the optical recording layer 10A. Since these object light 20L and reference light 30R have their agreed polarization directions, an interference pattern is produced by interference.
Accordingly, in the optical recording layer 10A, the interference pattern due to the interference of the B polarized object light 20L before entering into the reflection plane 10D with the B polarized reference light 30R after having been reflected at the reflection plane 10D and the interference pattern due to the interference of the B polarized reference light 30R before entering into the reflection plane 10D with the B polarized object light 20L after having been reflected at the reflection plane 10D are recorded in a three-dimensional manner.
(II) Then, the principle of reproducing data by using the two-parted optical rotation plate 100 will be explained with reference to FIG. 18.
The reproducing light 40R having passed through the two-parted optical rotation plate part 100R to enter into the optical recording medium 10 becomes a B polarized light. On the other hand, the reproducing light 40L having passed through the two-parted optical rotation plate part 100L to enter into the optical recording medium 10 becomes an A polarized light.
In the optical recording layer 10A, a data light traveling in the opposite direction with respect to the reflection plane 10D is generated by the reproducing light before being reflected at the reflection plane 10D, and a data light traveling toward the reflection plane 10D is generated by the reproducing light after having been reflected at the reflection plane 10D.
The data light traveling in the opposite direction with respect to the reflection plane 10D emits straightly from the optical recording medium 10, and the data light traveling toward the reflection plane 10D emits from the optical recording medium 10 after having been reflected at the reflection plane 10D.
The data lights are rendered to be parallel light beams by the objective lens 118 to be incident into the two-parted optical rotation plate 100. Here, the data light 50R incident into the two-parted optical rotation plate part 100R in the two-parted optical rotation plate 100 is a B polarized light before being incident into the two-parted optical rotation plate part 100R and becomes a P polarized light after having passed through the two-parted optical rotation plate part 100R. On the other hand, the data light 50L incident into the two-parted optical rotation plate part 100L in the two-parted optical rotation plate 100 is an A polarized light before being incident into the two-parted optical rotation plate part 100L and becomes a P polarized light after having passed through the two-parted optical rotation plate part 100L. Thus, the data lights after having passed through the two-parted optical rotation plate 100 become P polarized lights in the entirely cross-sectional view of the light beams.
The data lights having passed through the two-parted optical rotation plate 100 enter into the polarizing beam splitter 116, pass through the polarizing beam splitter plane 116A, pass through the beam splitter plane (BS), an image focusing lens and so on which are not shown, and enter into an image pickup element such as CCD to be converted into electric image signals.
Explanation will be made with reference to the cross-sectional view of FIG. 20 with respect to the structure of the polarizing beam splitter (PBS) used as a beam combining means for aligning the optical axes of an object light and a reference light, as an example.
In prisms 101, 102 each formed by processing a transparent glass block into a light isosceles triangular prism-like shape, a multilayered film 103 is formed on the inclined plane of the prism 101 to form a polarizing beam splitter plane wherein the multilayered film is formed by laminating alternately a thin dielectric film having a relatively large refractive index and a thin dielectric film having a relatively small refractive index in a film thickness of an order of wavelength. Then, the inclined surfaces of the prisms 101, 102 are bonded using a transparent homogeneous adhesive 104 to form a rectangular prism-like polarizing beam splitter PB5.
By adjusting the film thickness of the multilayered film 103, the polarizing beam splitter which reflects an S polarized light component having the polarization direction along the surface of the multilayered film 103 with respect to an incident light of wavelength λ and allows to pass through a P polarized light component having the polarization direction perpendicular to the polarization direction of the S polarized light component, can be formed. Further, by adjusting the film thickness of the multilayered film 103, the reflectance of the S polarized light component and the transmittance of the P polarized light component can also be adjusted.
Further, explanation will be made with reference to FIG. 9 with respect to an optical data recording/reproducing device D4 employing a plurality of polarizing beam splitters PB5 in combination.
A linearly polarized light (S polarized light) emitted from a light source 111 capable of emitting a coherent laser light (an interfering light) is rendered to be parallel light beams by using a collimator lens 112. The parallel light beams are introduced into, for example, a half-wave plate 113 as an optical rotation element to be converted into light beams including an S polarized light component and a P polarized light component wherein the ratio between the S polarized light component and the P polarizing light component to be introduced into a first polarizing beam splitter (PBS) 114 is adjusted.
The P polarized light component having passed through the first polarizing beam splitter 114 is introduced into a first beam splitter (BS) 115.
The first beams splitter (BS) 115 is adapted to allow, for example, 20% of P polarized light component to pass straightly therethrough and to reflect (at a direction of 90°) 80% of P polarized light component. A second polarizing beam splitter 116, a two-parted optical rotation plate 100 and an objective lens 118 are located in this order from the side of the first beam splitter 115 along the traveling direction of light, emitted from the light source 111, after having been reflected at the first beam splitter 115.
Further, a spatial light modulating element 119 and a second beam splitter (BS) 120 are located in this order from the side of the first polarizing beam splitter 114 in this optical data recording/reproducing device, in the traveling direction of the S polarized light component reflected at the first polarizing beam splitter 114 after having passed through the half-wave plate 113.
The spatial light modulating element 119 comprises a large number of pixels arranged in a lattice-like form wherein each pixel is capable of passing light or blocking light selectively whereby light is modulated spatially depending on its intensity whereby it is possible to produce a data carrying object light.
When a liquid crystal device in which aligning directions of liquid crystal molecules change depending on a voltage applied to the transparent electrode formed in each pixel whereby the state of polarization of transmitting light is changed, is used as such spatial light modulating element 119, and a polarizing beam splitter capable of reflecting 100% of S polarized light component and allowing to pass through 100% of P polarized light component is used as the second beam splitter 120, the change in the state of polarization of the light passing through the liquid crystal device is preferably converted into the change of light intensity by this polarizing beam splitter.
Namely, the light passing through a pixel of the liquid crystal device in the state of the S polarized light which is the same as the state of polarization of the incident light is reflected at the second beam splitter 120 and is introduced into the second polarizing beam splitter 116. On the other hand, the light having passed through a pixel of the liquid crystal device in the state of the P polarized light does not enter into the second polarizing beam splitter 116 because it passes through the second beam splitter 120.
The second polarizing beam splitter 116 as a beam combining means reflects the S polarized light component of the incident light, as an object light from the second beam splitter 120 to deflect its traveling direction 90°; passes the P polarized light component of the incident light, as a reference light from the first beam splitter 115, and aligns the optical axis of the object light and the optical axis of the reference light to combine these lights to be introduced into the two-parted optical rotation plate 100 and the objective lens 118.
Thus, the light emitted from the light source 111 is separated into the P polarized light component and the S polarized light component by the first polarizing beam splitter 114 to produce respectively the reference light and the object light and the P polarized light component and the S polarized light component are combined by the second polarizing beam splitter 116 (PBS), and then, the combined lights pass through the two-parted optical rotation plate 100 and the objective lens 118 to be focused on the optical recording medium 10.
Further, in order to reproduce the data of interference pattern recorded in the optical recording medium 10, an image focusing lens 121 and an image pickup element 122 such as CCD are located in this order from the side of the first beam splitter 115, which is at the opposite side of the second polarizing beam splitter 116 with respect to the first beam splitter 115. With this arrangement, only the reproducing light is irradiated to the optical recording medium 10; the data light reproduced at the medium passes through the second polarizing beam splitter 116; a part of the light passes through the first beam splitter 115 and enters into the image pickup element 122 through the image focusing lens 121. Thus, the data of interference pattern produced in the spatial light modulating element 119 and recorded in the optical recording medium 10 can be reproduced by the image pickup element 122.
Now, explanation will be made with reference to FIG. 21 on an embodiment of a flat plate type polarizing beam splitter PB6 capable of reducing the size of the device, in comparison with the rectangular prism-like polarizing beam splitter PB5 formed by bonding two prisms.
A polymer liquid crystal layer 106 having an ordinary refractive index no and an extraordinary refractive index ne is formed by forming an alignment layer on a single surface of a transparent substrate 105, conducting an aligning treatment, coating liquid crystal monomer on the film followed by polymerizing.
Then, the polymer liquid crystal layer 106 is subjected to photolithography and reactive ion etching to process it to have a blazed grating having a saw-tooth-form in cross section and a grating constant (pitch) L. Then, a homogeneous refractive index transparent material having the same refractive index as the ordinary refractive index no is filled in concave portions of the polymer liquid crystal layer 106 to thereby form a homogeneous refractive index transparent material 107, and at the same time, the transparent substrate 105 and a transparent substrate 108 are bonded together.
When a P polarized light as an ordinary polarized light is incident into the polymer liquid crystal layer 106, the light passes straightly through it without diffraction since the refractive index of the polymer liquid crystal layer 106 agrees with the refractive index of the homogeneous refractive index transparent material 107. On the other hand, when an S polarized light as an extraordinary polarized light is incident into the polymer liquid crystal layer 106, a diffracted light is generated since the refractive index of the polymer liquid crystal layer 106 is different from the refractive index of the homogeneous refractive index transparent material 106.
For example, the height d of the saw-tooth-like polymer liquid crystal layer 106 satisfies formula (ne−no)×d=λ with respect to an incident light of wavelength λ, the maximum diffracted light generates in a direction of angle θ satisfying sin θ=λ/L (where L is a grating constant (pitch)). In this way, the flat plate type polarizing beam splitter PB6 is obtainable.
In the above-mentioned Patent Document 1 disclosing the optical data recording/reproducing device, there is found explanation relating to the function of the two-parted optical rotation plate 100. However, it fails to disclose a concrete structure of the device. Further, the two-parted optical rotation plate 100 whose principle of operation is shown in FIGS. 17 and 18 has the structure that the two-parted optical rotation plate part 100R and the two-parted optical rotation plate part 100L are adjacently located. Even though a person skilled in the art can consider to arrange a wavelength plate using a birefringent material such as quartz or the like, no device usable practically was disclosed. In such circumstances, there is a strong demand to develop an optical device (phase plate) of small size and light weight and having regions defined highly accurately so as to perform the same function as the two-parted optical rotation plate.
Further, it is effective to form an interference pattern in a specified region of an optical recording medium 10 by applying to an optical data recording/reproducing device a focus servo method and a tracking servo method used for an optical head device performing data-recording/reproducing for an optical disk such as CD, DVD or the like.
For example, a beam splitter having wavelength selectivity (not shown) is located as a color beam combining means in the light path between the polarizing beam splitter 116 and the two-parted optical rotation plate 100 in FIG. 17 wherein the beam splitter being capable of passing light beams having a wavelength λ as the before-mentioned object light and reference light therethrough and reflecting light beams having a wavelength λs (λs≠λ) which is different from the wavelength λ and which is not photo-sensitive to the optical recording medium 10, and the optical axis of the incident light of wavelength λs is made coincident with the optical axis of the incident light of wavelength λ so as to focus the light beams into the light reflection layer of the optical recording medium by means of the objective lens 118. The light of wavelength λs reflected at the light reflection layer is separated by the above-mentioned beam splitter having wavelength selectivity to be detected by a photodetector.
However, when the light of wavelength λs passes through the two-parted optical rotation plate 100, the focused light spot is expanded because the polarized light is not uniform in traveling spatially whereby there causes the problems that the conventionally used focus servo method and the tracking servo method can not be applied.
Further, in order to control light paths of the transmitting light and the reflected light with good reproducibility and high accuracy in the above-mentioned rectangular prism-like polarizing beam splitter PB5 shown in FIG. 20, it was necessary to process a glass block accurately to have a right isosceles triangular prism form and to bond and fix two prisms with good accuracy. It resulted an expensive optical component, and when a plurality of polarizing beam splitters and beam splitters were used in the optical data recording/reproducing device D4 shown in FIG. 9, the cost was increased and there was the problem that the adjustment of optical axes was difficult.
Further, in the case of the flat plate type polarizing beam splitter PB6 shown in FIG. 26, the angle of separation θ by the diffraction of the P polarized light and S polarized light relied on the processable grating constant (pitch) L, specifically, about 4 μm. Accordingly, the angle of separation θ with respect to visible light restricted to about 10°. As a result, when the flat plate type polarizing beam splitter PB6 was used in the optical data recording/reproducing device as shown in FIG. 9, the entire optical system became large in size in order to ensure the irradiation planes for the object light and the reference light.